1. Field of the Invention
The present invention relates to a method of preparing a hearth roll with a coating. More particularly, the present invention relates to a method of thermally spraying electroless-plated ceramic powder and metal powder simultaneously onto a hearth roll.
2. Description of the Related Arts
In the process of continuous annealing and continuous galvanization, a rolled steel strip is conveyed by means of a hearth roll. Conventionally, a hearth roll is installed in a high temperature zone (800-1200.degree. C.), thus, in addition to having thermal oxidation resistance, the surface of a hearth roll has to retain hardness under high temperature, thereby maintaining the necessary surface roughness to produce the necessary friction to convey and avoid the slippage of a steel strip. In addition, there is also a need for a hearth roll to have excellent chemical inertness in order to prevent ferric oxide or other metal oxides of the surface of a steel strip from adhering to the surface of a hearth roll to form the buildup and cause defects on the surface of a steel strip.
An earlier technique to prevent the formation of the buildup on the surface of a hearth roll was to plate hard chromium on the surface of a hearth roll to form a hard layer. However, such hard chromium layer has the disadvantage of decreasing in hardness when the temperature is higher than 450.degree. C. Thus, recently a new technique has been developed, in which heat-resistant alloy powder with high hardness or ceramic powder are melted, thermally sprayed, and thus adhered to the surface of a hearth roll. The alloy coating has the advantages of easy-fabrication and an excellent adhesion; however, it is restricted by insufficient hardness under high temperature. In contrast, the ceramic coating has the advantages of hardness under high temperature, heat-resistance and wear-resistance; however, due to the poor adhesion, it has the disadvantage of easy-collapse under high temperature, and thus the surface roughness cannot be maintained.
Therefore, the cement coating technique, with the advantages of both alloy and ceramic coatings, has been widely used in the treatment of a hearth roll recently. The technique used is to mix ceramic powder and metal powder first; afterward, the mixed powder was sprayed simultaneously onto the surface of a hearth roll so that this coating has the advantages of both excellent adhesion from the alloy coating and hardness under high temperature from the ceramic coating.
The technology of the cement coating is described in U.S Pat. No. 4,470,802. Niobium powder (metal powder) and inorganic material (ceramic powder) are mixed mechanically, and then the mixture is sprayed onto the surface of a hearth roll.
However, this process has the following disadvantages. During the period of storage and transportation, the metal powder tends to settle due to the different densities between metal powder and ceramic powder. Thus, when the powder for spraying is obtained by mixing the two powders mechanically, the ratio between metal powder and ceramic powder will be changed. Thus, the different densities between metal powder and ceramic powder can cause the loss of the ceramic powder during the spraying, making the hardness of the coating insufficient. Moreover, because of the different surface tensions between metal powder and ceramic powder, the melted metal powder has insufficient wettability to the ceramic powder, so that the coating will have higher porosity and lower buildup-resistance.